1. Field of the Invention
The subject invention relates to novel secondary amine-terminated siloxanes, and to a novel and efficient process for their preparation. These products are useful in polymer preparation and modification, and in particular, as difunctional amine curing agents for epoxy and other aminereactive resins.
2. Description of the Related Art
Epoxy resins are widely used in fabricating structures, in particular as adhesives and as matrix resins in heat curable, thermosetting, fiber-reinforced composites. Epoxy resins are suitable for such applications because of their excellent physical properties. However, epoxy resins which maintain high tensile strengths at elevated temperatures, such as the amine cured polyglycidyl derivatives of p-aminophenol or 4,4'-methylenedianiline, tend to be somewhat brittle. Thus, while their high tensile strengths make them particularly compatible with high strength fibers such as carbon/graphite, glass, aramid, and polyethylene, their brittle nature causes them to be subject to considerable impact-induced damage.
Epoxy resins used as adhesives are generally of lower functionality and/or higher equivalent weight, and therefore have less cross-linking density and lower tensile strength that their higher-functionality, matrix-resin kindred. However, even here, increased resistance to impact damage would be desirable.
Functionalized elastomers such as the amino- or carboxy-terminated butadiene-acrylonitrile copolymers (ATBN and CTBN, respectively) available from B.F. Goodrich Corp. under the trademark HYCAR.RTM. have been used with some degree of success in toughening both adhesive and matrix resin formulations. See, for example, the article by J. Riffle, et. al., entitled "Elastomeric Polysiloxane Modifiers" in Epoxy Resin Chemistry II, R. Bauer, Ed., ACS Symposium Series No. 221, American Chemical Society, and the references cited therein.
The use of ATBN elastomers having carbon backbones containing unsaturation, while increasing toughness, does not provide sufficient thermal and/or oxidative stability for many modern applications of adhesives and matrix resins, particularly those in the aerospace field. Thus it has been proposed to utilize functionalized polysiloxanes for these applications, relying on the thermal-oxidative stability of the silicon-containing backbone to lend increased thermal stability to the total resin system. Several such approaches have been discussed in Riffel, supra, and involve primary amine terminated polysiloxanes such as bis(3-aminopropyl)polysiloxanes and secondary amine terminated polysiloxanes such as bis(piperazinyl)polysiloxanes.
Perhaps due to their lower functionality, the secondary amine terminated, piperazinyl polysiloxanes generally proved to have superior physical properties than the primary amine terminated polysiloxanes (tetrafunctional). Unfortunately, these secondary amine terminated polysiloxanes are difficult to prepare.
One preparation of piperazinyl functionalized polysiloxanes involves reaction of 2-aminoethyl piperazine with a previously synthesized carboxy-terminated polysiloxane to form the bis(2-piperazinyl ethyl amide) of the polysiloxane: ##STR1##
A second approach is to react a large excess (to avoid polymer formation) of piperazine with a bis-epoxy polysiloxane, producing a bis(2-hydroxy-3-piperazinyl) polysiloxane: ##STR2## This method, of course, requires prior preparation of the epoxy-functional polysiloxane.
Ryang, in U.S. Pat. No. 4,511,701, prepared both primary and secondary amine-terminated polysiloxanes by reacting an appropriately substituted diamine with difunctional silylnorbornane anhydrides, themselves prepared as disclosed by Ryang in U.S. Pat. No. 4,381,396. Reaction of these diamines with the bis(anhydride) functional polysiloxanes results in amino-imides such as: ##STR3##
Only the last-mentioned process produces aminofunctional polysiloxanes which are truly difunctional. The amide hydrogen and hydroxyl hydrogen produced by the first two preparations, though less reactive than the secondary amino hydrogens, are nevertheless reactive species in most resin systems. Their presence, therefore can cause further, and at times unpredictable crosslinking, either over an extended period of time in normal service, or as a result of high curing temperatures.
Furthermore, all of the foregoing preparations involve many steps, and in the process consume large quantities of relatively expensive chemical reagents. All these prior art products are difficult to prepare, expensive products, and thus there remains a need for thermally stable, secondary amine terminated polysiloxanes which may be prepared in high yield and in an economic manner.